Ex Katia - UK Bound. Forecasts and in-depth discussion

[Liam J]

GFS 00z latest run appears to have shifted the track slightly south, and a tighter center, the southern flank of strongest belt of damaging winds looks to be a little further south.

need an expert on this and how significant it would be.

first chart monday 0000hrs second is 0900hrs

purple lines indicate track.

Yes the 00z has made a minor adjustment southwards and N England is back into the firing for damaging gusts, N Ireland and Scotland still on course for very severe weather.

The 06z might take her north again but we shall see, less than 48 hours to go now...

Yes the 00z has made a minor adjustment southwards and N England is back into the firing for damaging gusts, N Ireland and Scotland still on course for very severe weather.

The 06z might take her north again but we shall see, less than 48 hours to go now...

Meto this morning -

Stormy weather heading for the UK

The forecast team at the Met Office are continuing to keep an eye on the remains of Hurricane Katia as it moves across the Atlantic Ocean. Currently the storm lies just south of Newfoundland in Canada, but it is expected to move towards northwestern parts of the UK through the course of this weekend, arriving some time on Monday morning. Please see our warnings page for further details. Issued at 0612 on Sat 10 Sep 2011.

[Terminal Moraine]

GFS has consistently taken the centre of 'Katia' about 150 miles further north than the track shown on the Fax charts by the time she reaches Scotland, a crucial difference when it comes to the area affected by the strongest winds.

I would think it will be this time tomorrow before there's any degree of certainty over the exact track and perhaps even beyond that for minor adjustments.

In my area I'm expecting little more than a windy day, ie nothing exceptional. If I get a gust of 70 mph I'll consider it a job well done.

[Aaron]

Now the Met Office have us down for 53 mph gusts on Monday.. wouldn't be surprised to see it higher though.

[Iceberg]

There is still considerable differences between the models as to how this effects the BI.

To back up what Chino mentioned about the uncertainties we have a variance in the low pressure system that contains ex-Katia at T48 of

20mb between the UKMET and GFS with ECM somewhere inbetween, this is not normal by any standards.

Just for info here are the below forecasts for T48.

UKMET 974mb, NAE 972mb, GFS 954mb and ECM 962mb.

The UAE is not a global model and uses a UKMET feed ran at a higher resolution which is why it comes out so close to the UKMET.

There is little doubt that most of the variance comes from the initialisation of Katia still, which is currently tropical and warm core in nature.

Nobody knows the actual pressure of Katia which could be anywhere from 980 to 960 atm, equally nobody knows what the pressure gradients inside Katia looks like.

The knock on effects are shown best when looking at the wind isographs IMO. Here are the NAE and GFS, the GFS has a much large swave of very strong sustained winds( i.e 50mph and greater) and has winds upto 60-65mph sustained at 10m, this would easily give sustained winds of 70-80mph at elevation and maybe gusts to 100-110mph when you look at the 925mb winds.

NAE are on average 10-20mph lower, it doesn't sound much, but when the trees are in full leaf 20mph faster will make the difference between structural damage or not.

Katia has now currently lost her eye, but still has a CDO as it travels across waters only 20-22C it will nolonger be deriving any energy from the Ocean or from it's core dynamics and pressure will be filling, the strength of Katia near our shores will be determined by how quickly baroclinic transistion happens now, importantly NHC have upped her forward speed to faster than antipated this should make her stronger than thought nearer our shores.

[Coast]

All the US late model tracks are back to the North this morning. The thing to remember with them is though, they do not have a high degree of confidence in them (as shown in the latest NHC discussion) post 96 hrs from the US. Here they are anyway, to analyse post event:

[Coast]

GFS has most of Scotland covered and significant parts of the Midlands in a tornado area:

85 kts in Scotland down to 45 kts in the South

[Interitus]

Nobody knows the actual pressure of Katia which could be anywhere from 980 to 960 atm, equally nobody knows what the pressure gradients inside Katia looks like.

The early morning estimates from the NHC of 970mb proved to be a fair way out as the storm passed near to a Canadian buoy which recorded 957mb leading to a revised estimate of 954mb

The knock on effects are shown best when looking at the wind isographs IMO. Here are the NAE and GFS, the GFS has a much large swave of very strong sustained winds( i.e 50mph and greater) and has winds upto 60-65mph sustained at 10m, this would easily give sustained winds of 70-80mph at elevation and maybe gusts to 100-110mph when you look at the 925mb winds.

NAE are on average 10-20mph lower, it doesn't sound much, but when the trees are in full leaf 20mph faster will make the difference between structural damage or not.

Probably more significant than being in leaf is that the energy of the wind is proportional to the cube of it's velocity.

Hence a wind with 26% higher velocity from 60mph to a little over 75mph has twice the power.

[Iceberg]

Not been around this afternoon but the news that the system is nearly 20mb lower than thought is major news If not totally unexpected equally the news that the winds are further our and stronger than thought when back on a computer it will be worth seeing what the globals initialiaed her as on the 12z suite

[Coast]

Looks to be a tad lower according to GFS, but now seems to hang around for longer and becomes more severe as the day progresses::

Convective gusts around 80kts in the North West of Scotland isn't good:

Netweather's NMM model also has it arriving later and further South:

Gusts to over 80mph possible

Sustained winds of up to 56 mph?

NMM not yet out far enough for later in the day, but well worth keeping an eye on if you have NW extra.

[Coast]

UKMO chart from 21st OWS has sustained winds maxing further down too:

They also have two advisories out for today with 34knot winds before things kick off tomorrow:

within hrs

Weather Advisory 09-010 for Fairford (EGVA) Valid 11/0845Z (11/0945L) to 11/1800Z (11/1900L)

Winds from 25 to 34 kts. forecast value 25 kts.

Winds are from the south west.

within hrs

Weather Advisory 09-017 for Mildenhall (EGUN) Valid 11/0900Z (11/1000L) to 11/1500Z (11/1600L)

Winds from 25 to 34 kts. forecast value 25 kts. is forecast

Winds are from the south west.

[Rob]

It's very interesting that the NMM shows the system arriving later and further south that the other models, it would tie in well with the Met Office extending the warning southward, perhaps the NMM is on to something here?

Edited September 11, 2011 by Rob

[Paul]

Latest on Katia now online:

http://www.netweather.tv/index.cgi?action=news;storyid=801;sess=

[Coast]

It's very interesting that the NMM shows the system arriving later and further south that the other models, it would tie in well with the Met Office extending the warning southward, perhaps the NMM is on to something here?

Possibly....

Things don't really get going until later on the latest NMM info:

So she could still spring a few surprises on us. 56mph sustained winds and gusts up to 90mph on the cards for Ireland and exposed parts of Scotland down to the far NW of England

The jet does indeed seem to making an impression on it's new cross Atlantic girlfriend, Katia:

Incredible shear late afternoon - tornado alley in Scotland and Northern areas of England?

[Staffordshire]

Just wondering if any squall lines will develop and where?

[Coast]

Even there on Tuesday early morning!!!

[knocker]

It seems to me that the consensus model tracks from a few days ago weren't far out. Some perhaps, METO?, had it slightly deeper. Midday position.

Edited September 12, 2011 by weather ship

[Coast]

The US model tracks weren't too bad and remained consistent with the majority getting within 100miles or so of the centre. Quite encouraging that the windspeeds weren't far out in the end either as NHC didn't hold out for accuracy of track or speeds so far from US waters: